See page 39 about the difference between the "public clone url" and "the clone url". The public one is READ-ONLY. See example on page 44.

More details: "He made the mistake to copy the public clone URL to clipboard and not his own URL. So he forked the original master. And as long as you are not added to the list of collaborators from the project's owner, you are not allowed to push directly to his REPO.When you fork, the project's owner can decide whether to apply your changes or not. So he is in control, if you are actually not yet a trustworthy collaborator."